History has never felt this cool. Urban explorer Bob Diamond is gearing up for his next tour of the once forgotten Long Island Railroad tunnel, 30 feet below Atlantic Avenue between Hicks and Court streets – where the temperature is always a comfortable 65 degrees.

The tours have been captivating crowds 200 strong all summer, said Diamond, the president of the Brooklyn Historic Railway Association.

“It makes me feel really happy when people come down in the tunnel and the first thing out of their mouths is, ‘This is awesome!'” he said. “That’s what keeps me going down there.”

The tunnel, built in 1844 as a route between New York Harbor and Boston, was sealed up and abandoned in 1861. The structure, and Diamond, are the subjects of a planned feature-length documentary. The documentary has attracted the attention of several major networks, including The National Geographic Channel, according to Jerry Kolber, the film’s producer. Diamond’s belief is that an old steam locomotive is buried within one of the tunnel walls, and the filmmakers hope to chronicle its unearthing.

Tales of the tunnel’s ignoble past – river pirates were said to store their booty there, and German saboteurs were thought to have manufactured mustard gas there during WorldWar I – continue to attract throngs.

The jaunts have so far drawn visitors from 26 states and eight different countries, Diamond said. “A month ago we had a group of Germans along for the tour,” Diamond said. “They liked the story about the German saboteurs.”

The next public tour of the tunnel will be on Sunday, August 16. Call 718-941-3160 for reservations and information. Flashlights and comfortable shoes are recommended. For more information about the BHRA, go to www.brooklynrail.net.